Degradation Process of Herbicides in Biochar-Amended Soils: Impact on Persistence and Remediation

Biochar is a solid material derived from different feedstocks that is added to the soil for various agronomic and environmental purposes, such as nutrient sources and CO 2 emission mitigators. In modern agriculture, the application of herbicides directly in the soil is common for pre-emergent weed control; however, biochars may interfere in the degradation processes of these agrochemicals, increasing or decreasing their persistence. Long persistence is desirable for some herbicides in determined cultivation systems, especially in monoculture, but persistence is unde-sirable in crop rotation and/or succession systems because the subsequent cropping can be sensitive to the herbicide, causing carryover problems. Therefore, knowing the interactions of biochar-herbicide is essential, since these interactions depend on feedstock, pyrolysis conditions (production temperature), application rate, biochar aging, among other factors; and the physical-chemical characteristics of the herbicide. This chapter shows that the addition of biochar in the soil interferes in the persistence or remediation processes of the herbicide, and taking advantage of the agricultural and environmental benefits of biochars without compromising weed control requires a broad knowledge of the characteristics of biochar, soil, and herbicide and their interactions.


Introduction
Herbicides are the pesticides most applied in modern agriculture for weed control worldwide, in pre-emergency, directly in the soil, or in post-emergence in leaves. Regardless of the application of herbicides, these reach the soil and may persist with residual effect (carryover) or contaminate the non-target organism and environment. The behavior of the herbicide in the soil is governed by the physicochemical properties of the molecule and the soil and can have retention, transport, and transformation processes [1]. In transformation processes, the herbicide molecule is degraded into secondary compounds (metabolites) by physical (photodegradation), chemical, and biological processes (Figure 1) [2].

Microbial diversity in biochar-amended soils
Soil correction with biochar can affect the soil microbiota in different ways: (1) It can provide an increase in the microbiota [23,24]; (2) It can negatively affect the resident microbiota by the amount of organic substances (volatile compounds) formed in the production of biochar [25,26]; or (3) It may not effect the soil microbiota [27,28]. The possible interaction mechanisms of biochar and soil microbiota are exemplified in Figure 2 [29,30]. The physical-chemical structures of the biochar surface (macro and micropores, roughness, surface load, and hydrophobicity) are a refuge for the soil microbiota [31,32], where microorganisms can find nutrients and ions adsorbed in biochar particles useful for their growth [29,33]. In addition, biochars can contain significant amounts of organic substances (volatile organic compounds and free radicals) [34,35], improve the soil's physical-chemical properties, which are important for microbial growth by modifying habitats (aeration, water content, and pH) [36], affect the enzymatic activity of the soil [37,38], and increase the sorption of herbicides, reducing the bioavailability and toxicity of these agrochemicals for the soil microbiota [29,39,40].
Biochar-amended soil has a higher respiratory rate and microbial communities due to carbon mineralization by soil microorganisms [41]. Microbial biomass carbon and nitrogen increased by 18% and 63% with the application of 1% of sugarcane bagasse biochar [42]. The role of biochar nutrients in the biodegradation of coexisting dichlobenil and atrazine in soil by their respective bacterial degraders was evaluated. The degradation increased with increasing biochar content, due to nutritional stimulation on microbial activities [43]. The application of hardwoodderived biochar increased atrazine mineralization by stimulating atrazine-adapted microflora compared to unamended soil [19]. Soil amended with biochar derived from wheat straw increased the abundance and diversity rate of bacteria and fungi beneficial to plants in the rhizosphere of wheat seedlings [24]. In addition, these microorganisms use fomesafen as a source of nutrients, which favors their proliferation from the soil [24]. The change and proliferation of the soil microbiota with the addition of biochar is related to the chemical characteristics of biochar (mainly pH and nutrient content) and physical properties (pore size, pore-volume, and specific surface area), OM content, and water retention that provide favorable conditions for soil microbiota [28]. Although soil microbial biomass is generally benefited with the addition of biochar, the response depends on the type of raw material, pyrolysis temperature, and biochar application rate, since these factors directly interfere with the physical-chemical characteristics of biochar and consequently on the response of the microbiota in herbicide degradation. The proposed mechanisms involved in biochar and microbiota interactions require further studies to elucidate the impact of biochar on soil microbial activity.

Influence of biochar amendment in soil on the herbicide degradation
Herbicides are applied to the soil to control weeds during a certain time after application; however, long persistence may affect the subsequent crop, a process known as carryover. Therefore, the process of degradation of the herbicide is Interactions between biochar and soil microbiota and environmental effects. Source: Adapted from Zhu et al. [29].
Degradation Process of Herbicides in Biochar-Amended Soils: Impact on Persistence and… DOI: http://dx.doi.org /10.5772/intechopen.101916 important for the dissipation of herbicides in the soil when the intention is the remediation of the product. However, under agronomic conditions in which a residual effect of the herbicide on the soil is desired for weed control, the addition of biochar can reduce the persistence of the product, consequently reducing its effectiveness in management [2, 9,44].
The degradation of herbicide molecules into secondary compounds (metabolites) can occur by biotic (biological degradation) or abiotic (hydrolysis, reduction, oxidation, and photolysis) processes [45]. Biodegradation, carried out by the soil microbiota (bacteria, fungi, protozoa, and actinomycetes), is the main decomposition pathway for most herbicides [13,46]. Microorganisms can use herbicide molecules as an energy source and transform them into compounds without herbicide action, a process known as catabolism, or through co-metabolism, in which herbicide degradation requires the presence of a growth substrate that is used as primary carbon and energy source [3,47], i.e., microorganism does not obtain energy or benefit from the herbicide degradation. The transformation process is usually mediated by non-specific enzymes that are capable to transform various organic compounds [4]. Herbicides have varied susceptibility to microbial degradation depending on the complexity of the molecule that influences low or high persistence in the soil [5].
Microbial degradation generally reduces the DT 50 of herbicides in the soil; however, the addition of biochar, according to studies performed, may increase or decrease the DT 50 values, depending on the herbicide and pyrolyzed feedstock ( Table 1). The high sorption capacity for herbicides in the biochar-amended soil decreases herbicide degradation, providing a higher DT 50 than the unamended soil [46,49]. For example, less atrazine degradation was observed in amended soils with sugarcane bagasse biochar (0.5% w/w) ( Table 1), increasing in 15 days the DT 50 of the herbicide in relation to unamended soil [49]. Flumioxazin DT 50 increased by ~10 days when bamboo biochar (10% w/w) was added compared to unamended soil (Table 1) [51]. The DT 50 of 2-methyl-4-chlorophenoxyaceticacid (MCPA) increased from 5.2 days (unamended soil) to 21.5 days in amended soil with 1% of wheat straw biochar [59].
The application of biochar can also increase soil microbial activity, improving herbicide degradation [29,60]. The increase in microbial biomass may be due to the addition of available organic substrates, which are the main energy source readily available to soil microorganisms [55]. The high content of dissolved OC in the soil MO can reduce herbicide sorption by biochar particles, as dissolved OC competes with herbicide molecules to occupy available biochar sorption sites [61]. Biochar also sorbs dissolved OC, which can contribute to co-metabolic biodegradation [22]. Some changes in degradation rate may result from indirect effects of biochar amendment, e.g., changes in soil pH and aeration [18]. The highest degradation of oxyfluorfen was observed in amended soils with different rates of application of rice husk biochar, decreasing DT 50 between 2 and 23 days compared to unamended soil ( Table 1) [50]. Alachlor mineralization increased up to 50% using biochar derived from soybean stoves, sugarcane bagasse, and wood chips compared to unamended soil (Table 1) [58].
Photolysis and hydrolysis are the main abiotic processes involved in herbicide degradation [48,62]. Photolysis or photodegradation occurs when herbicides are exposed to sunlight [63] and can be direct (a herbicide molecule absorbs light energy, is later excited and transformed) or indirect (species photochemically produced in the soil matrix react with the herbicide molecule triggering its degradation) [64]. Water degrades herbicides by dividing large molecules into smaller molecules, breaking them in the process called hydrolysis [65]. The hydrolysis of herbicides in the soil can be influenced by several factors such as dissolved ion    concentration, soil pH, and content of clays and metal oxides capable of catalyzing this herbicide degradation process [14,66]. The application of biochar can influence the degradation of herbicides by hydrolysis and photolysis, since persistent free radicals existing or photogenerated in biochars can react with the herbicide by the activation of other free radicals such as hydroxyl, sulfate, anion, and superoxide [67,68]. In addition, the increase in soil pH, the presence of active groups on the mineral surface of biochar, and the high sorption of herbicides have a direct effect on the chemical degradation processes of herbicides [64,66]. Atrazine was hydrolyzed by 27.9% in the presence of biochar derived from pig manure (700°C) after 12 h due to the mineral surface and dissolved metal ions released from biochars that catalyze hydrolysis [66]. In contrast, imazapic and imazapyr were resistant to degradation by hydrolysis in amended soil with biochar derived from empty fruit bunch of oil palm and rice husk, and their DT 50 's increased by ~6 to 12 days because the photodegradation rate diminished [53] ( Table 1). The addition of biochar to the soil at 1 or 5% inhibited the photodegradation of metribuzin and its metabolites deamino (DA), deaminodiketo (DADK), and diketometribuzin (DK), which increased their DT 50 's due to the immobilization of these compounds the surface layer of the biochar [64]. Therefore, the application of biochar has a direct impact on herbicide degradation processes and should be constantly examined for its application in the soil.

Factors affecting herbicide degradation in biochar-amended soils
The impact on the degradation of herbicides due to their high sorption in the biochar particles depends on the rate of biochar applied to the soil. The application of different rates of application of hardwood biochar in Rhodic Ferralsol soil increased atrazine degradation by 49% (0.1% of biochar), 51% (1.0% of biochar), and 62% (5.0% of biochar) after 88 days of incubation (Table 1) [19]. DT 50 of isoproturon in unamended Alfisol was 16 days, however, when biochar was added at 1.5 and 5%, DT 50 increased to 67 and 136 days, respectively (Table 1) [46], i.e., the persistence of isoproturon is prolonged as the rate of biochar added to the soil increases. DT 50 of fomesafen increased from 34.6 days in unamended soil to 51, 83, and 160 days in amended soils with rice husk biochar at 0.5, 1, and 2%, respectively [61]. The increased persistence of fomesafen can be explained by the higher sorb capacity of biochar and, therefore, little bioavailability of the herbicide for microbial degradation.
Pyrolysis temperature defines the physicochemical characteristics of biochars [69]. Generally, biochar produced at relatively high pyrolysis temperatures (>500°C) presents an increase in specific surface area, microporosity, and hydrophobicity, improving herbicide sorption [70]. However, even with higher herbicide sorption capacity, degradation at high pyrolysis temperatures may be more intensified than low temperatures. The addition of sugarcane bagasse biochar produced at 700°C in clay soil decreased the DT 50 of metribuzin from 57 (unamended soil) to 39 days, but when biochar was produced at 350°C, DT 50 went from 57 to 74 days (Table 1) [56]. These conflicting results could be due to the impact of ash on the alkalinity of the soil amended with biochar produced at 700°C (20.3% of ash), which increased the soil pH and improved the conditions for the degradation of metribuzin, and to the greater amount of dissolved OC from biochar produced at 350°C (3.78 mg g −1 ), which is more preferred by microorganisms as substrate, increasing the persistence of the herbicide. The variation in pyrolysis temperature of eucalyptus wood residue biochar affected the total hexazinone unavailable (mineralized + non-extractable residue) being higher for 850°C (46%) and 950°C (49%) compared to biochar pyrolised at 650°C (33%) and 750°C (42%) [71]. The addition of biochar did not alter the mineralization of hexazinone, but it did reduce the bioavailability of this herbicide in the soil due to the greater amount of nonextracted residue, reducing the risk of environmental contamination [71].
Aging alters the properties of biochar, affecting the degradation of herbicides, however, these changes are not fully elucidated [72]. Glyphosate showed no variation in degradation in two tropical soils (Ultisol and Alfisol) amended with eucalyptus biochar aged [73]. The aging of soil-wood biochar mixtures (Betula sp. and Piceaabies) decreased glyphosate and diuron sorption compared to fresh biochar amended soil [18]. In addition, herbicide degradation was not affected by changes or biochar aging in the soils studied [18]. The degradation of S-metolachlor was not affected with the addition of three macadamia nutshell biochars aged [74]. The persistence of mesotrione in different soils amended with fresh and aged biochar was similar to unamended soils [75]. In contrast, the extractable amounts of picloram were 20 and 50% lower for soils amended with fresh and aged oak wood biochar, respectively, in relation to unamended soil [76]. The addition of 10% fresh biochar from the olive oil industry increased the DT 50 of metribuzin from 20 (unamended soil) to 30.2 days, however, the DT 50 decreased to 6.4 days with the addition of aged biochar, possibly because microorganisms in soil aged with biochar used metribuzin as a source of carbon and energy instead of the labile fraction of soil OM (Table 1) [55]. The effects of biochar on herbicide degradation in soils should not be generalized due to the different characteristics of biochars and the complexity of the soil system. The variation of temperature and application rate of biochar can bring different degradation responses for each herbicide studied. Furthermore, the aging of biochar in the soil can influence the bioavailability of herbicides in soil solution by altering the sorption capacity of the biochar; therefore, the conditions of pyrolysis, type of feedstock as well as aging must be taken into consideration when planning its use in agriculture and for soil remediation purposes [18].

Simultaneous use of herbicides and biochar
In an agricultural context, the property of biochar that offers potential for herbicide sorption (environmental remediation) can also decrease the efficacy of herbicides applied to the soil, influencing their bioavailability and susceptibility to leaching and consequently their degradation [77]. The bioavailability of diuron and microbial degradation was reduced in soils amended with rice straw biochar, which decreased the effectiveness of diuron to jungle rice (Echinocloa colona) control [78]. The addition of wheat straw biochar to the soil inactivated the herbicides atrazine and trifluralin, resulting in increased seed germination and biomass of annual ryegrass (Lolium rigidum). In this study, the efficacy of the herbicides for ryegrass control was achieved when the application doses were four times higher than recommended [44]. In a bioassay with Echinochloa colona, injuries 9 days after planting decreased with increasing application rates of rice straw biochar indicating that sorption of clomazone increased and directly influenced the bioavailability of herbicide in the soil [79]. The control efficiency of S-metolachlor was evaluated on green foxtail (Setaria viridis) in soil amended with wood biochar at different application rates (0, 0.5, 1, and 2%) [80]. S. viridis control at the highest application rate (2%) was lower than the other application rates evaluated, however, better than the control treatments (no herbicide) [80].
The biochar applied to soil also influences the soil physicochemical properties and the improved nutritional availability of these directly impacts crop growth and consequently weed growth [81]. Soil amended with walnut shell biochar DOI: http://dx.doi.org /10.5772/intechopen.101916 (5 Mg ha −1 ) for 4 years was evaluated for weed control [82]. Weed density was dramatically higher in biochar-amended soils (60-78%) compared to unamended soil, being related to increased nutrient availability and improvements in soil physicochemical properties such as cation exchange capacity (CEC), density and porosity, increased soil aeration, and water retention. The application of 2 Mg ha −1 of cow bonechar prevented weed control by indaziflam which is related to the increase of soil fertility, especially the phosphorus and carbon content, and to the increase of pH because it is a basic material [83]. In addition, goosegrass (Eleusine indica) and crabgrass (Digitaria horizontalis) accounted for about 99.7% of the entire weed community infestation [83].
On the other hand, the decrease in efficacy depends on the characteristics of the herbicide evaluated. The dose of pretilachlor to inhibit 50% of E. colona emergence and biomass was higher in soil amended with rice-husk biochar, however, the effectiveness of pendimethalin in controlling E. colona was not influenced by the application rate of biochar [84]. The effectiveness on metribuzin in soils amended with biochar was evaluated by White Junior et al. [56]. The addition rates of biochar did not alter Palmer (Amaranthus palmeri) emergence, and it is possible that the residual activity was sufficient to reduce germination at any rate of biochar [56].
The addition of biochar to soil increases the sorption of different herbicides and reduces their effectiveness, which may result in the need for higher herbicide application rates, additional application times, or more weed control operations required [85]. Residual herbicides, applied in pre-emergence, can not provide good weed control regardless of soil type after biochar application. This does not necessarily mean that biochar should be avoided, however, when biochar is applied to the soil, management practices need to be adjusted to obtain appropriate weed control [86].

Conclusions
Modifying soil characteristics with biochar is a world-renowned emerging practice for either environmental and/or agronomic purposes, and the benefits these carbonaceous materials brig to the soil are clear. However, the pyrolysis conditions for biochar production directly interfere with the physical-chemical properties of the produced material, which govern the biochar-herbicide interactions. If the objective is to apply the herbicide in pre-emergence after the addition of biochar in the soil, care should be taken, as biochar can decrease or increase the persistence of the chemical product, interfering in the effectiveness of weed control over time. On the other hand, if the objective is herbicide remediation in contaminated soils, the interference of biochar in the bioavailability of the herbicide in the soil solution to increase soil microbiological diversity should be known.